Traditionally, chilled-iron roll mantles have been used as the roll mantle of paper machine rolls, especially of calender rolls. These roll mantles are manufactured by casting grey cast iron or alloys of the same into a chill mould, whereby, by the effect of the chill mould, the outer face of the cast piece is cooled rapidly and becomes graphite-free white iron. The structure of white iron consists of hard iron carbides and of perlite. The white irons are hard, highly wear-resistant, and hard to machine.
A drawback of chilled-iron roll mantles is impact brittleness resulting from the material as well as sensitivity to sudden changes in temperature, for which reason it has been necessary to limit their use in applications in which the temperatures change. Moreover, owing to the process of manufacture, i.e. chill casting, the hard white layer becomes uneven, which results in a form error in the roll and in uneven distribution of temperature when the operating temperature rises. Owing to the carbides-containing microstructure of the surface layer, the wear of the roll face is uneven, which can be seen in the paper and which also necessitates re-grinding of the roll at intervals of about 6 months.
Also, the service life of the chill molds used for the casting is limited, and their cost of manufacture is high. Moreover, since several different kinds of chill molds are needed, high sums of capital are bound in them.
Moreover, owing to the high hardness of white iron, the machining of chilled-iron roll mantles is very difficult, and therefore the costs of manufacture of the rolls become high.
As is well known, attempts have been made to replace mantles of chilled-iron rolls by roll mantles of steel, in which the temperature limitations of the chilled-iron rolls do not occur.
Chilled-iron mantles cannot be used at high temperatures, e.g., of about 250.degree. C., and, moreover, when such mantles are used, strict limits are imposed on the rate of cooling/heating, e.g., from about 0.5.degree. to about 2.degree. C. per minute, and the difference in temperature between the inner face and the outer face of the roll must never be higher than from about 30.degree. to about 50.degree. C.
Unless the above limitations are complied with, thermal shock will cause damage to the chilled-iron rolls.
When steel mantles are used, these limitations of temperature do not occur, but other problems occur. For example, doctoring causes problems, because doctoring is considerably more precise, and the doctor is worn extensively when steel mantles are used. Moreover, the mantle of a steel roll is polished during operation, and the adhesion of paper to the roll is increased, with resulting problems in the threading of the web. Also, the ability of steel to attenuate oscillations is lower than that of cast irons.
From the prior art, such solutions are also known for roll mantles in which the surface of a cast-iron roll mantle has been re-melted while thereby producing white iron in the surface layer, i.e. the objective being to provide a process substituted for chill cast, whereby a microstructure similar to chilled-iron roll-mantles is produced.
One such process is described in the U.S. Pat. No. 4,452,647, wherein a process and a device are described for the manufacture of hard-faced cast-iron pieces, in particular of rolls such as rolls for steel industry or paper calendering rolls, as well as a roll, cylinder or equivalent manufactured by means of the process. According to this reference, the cast pieces, such as rolls and cylinders, are cast in sand or in an equivalent way so that, in connection with the solidification, a substantially crystal structure of grey cast iron is produced in them and that, after this stage, a re-melting treatment is carried out by making use of an electron jet or jets to produce a cast hard at the surface. Thus, in this prior art method, a treatment of re-melting of the surface is employed, which is carried out after the work piece had been machined close to the ultimate dimensions, the process of surface-treatment being an electron jet or-jets so as to produce a heating effect that can be controlled and machined precisely.
A second prior art solution in which the surface is treated by means of the electron-jet melting process to produce a hardened cementite-carbide surface layer is described in the U.S. Pat. No. 4,000,011.
A further prior art solution based on re-melting of the surface is described in DE Patent No. 3,640,131, which discloses a mantle that has a hard mantle face and a process for the manufacture of a roll with a hard mantle face in particular for paper machines, which roll is made of cast iron that has been cast in a sand mold and, after rapid cooling, on the roll, an outer carbides-containing layer is produced together with a grey core zone. In this prior art solution, the cast roll is pre-heated to temperatures of about 400.degree. to about 600.degree. C., and the pre-heated surface layer is heated rapidly beyond the temperature of the liquid successively until the desired local melt layer is reached, and this melt layer is cooled rapidly to produce a carbides-containing zone. Finally, the whole roll is cooled to the ambient temperature.
In all of the prior art solutions described above, attempts have been made to provide a carbides-containing microstructure in the surface layer of the roll mantle, in which structure particles of iron carbides, i.e. cementite, are present. These iron carbides are very hard, and, when the roll is used, the roll face is worn around the carbides, and the carbides remain on the surface as outwardly projecting peaks. The flaw produced by these peaks can also be noticed in the papers produced.
In the prior art solutions, in connection with the rolls that are heated, problems have also been caused by the variations in the shape of the roll mantle when the temperature changes. In particular when the structure of the roll mantle consists of two different materials, for example when the inner part is made of flake-graphite cast iron and when the mantle surface is made of white iron after chill cast, changes in temperature result in problems, because the properties of thermal expansion and thermal conductivity of these different layer are different and, moreover, the thickness of the layer varies. The variations in shape and dimensions of the roll mantle are especially problematic, because the paper quality is deteriorated.